1. Field of the Invention
The present invention relates to an optical sensor apparatus and a lithographic apparatus comprising the same. In an embodiment, the invention relates to an optical sensor apparatus for use in an extreme ultraviolet lithographic system.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In addition to EUV radiation, radiation sources used in EUV lithography typically generate contaminant material that is harmful for the optics and the working environment wherein the lithographic process is carried out. This is especially the case for an EUV source operating via a discharge produced laser induced plasma. Hence, in EUV lithography, a desire exists to limit the contamination of the optical system that is arranged to condition the beam of radiation coming from an EUV source. Furthermore, it may be desirable to be able to monitor the amount of contamination that is accumulating in the EUV system. Another desire may be to be able to monitor the amount of EUV energy generated by an EUV source.